5G Physical Uplink Control Channel (PUCCH) and Uplink Control information (UCI)

This is a new episode of our series, "5G Explained." In this video, we discuss uplink control information or UCI in 5G New Radio. We will look at its content, how it is encoded and modulated, then mapped to the 5G New Radio slot via the PUCCH or physical uplink control channel. We will look at examples of usage and how it can be used for scheduling requests.

Uplink control information or UCI is very different from downlink control information or DCI. You may recall from another episode of this "5G Explained" series that DCI carries scheduling information for downlink and uplink, among other pieces of information. UCI content is more limited as the base station is already aware of all scheduled transmissions. It includes acknowledgments for previously transmitted blocks, channel quality indicators, or CQI, which lets the base station make decisions about beamforming and scheduling, as well as scheduling requests. Those pieces of information can be combined and transmitted simultaneously. The UCI is carried by the PUCCH or physical uplink control channel. It can also be carried on the PUSCH if a PUSCH transmission is scheduled.

Depending on the content of the UCI, one or more of several formats can be used. There are two basic cases for transmissions: payloads smaller than two bits and payloads larger than two bits. Payloads smaller that two bits use format 0 or format 1. Such payloads represent one or two ACK/NAK or scheduling requests. Format 1 is a longer alternative to format 0, which is useful for power limited or cell edge scenarios. Payloads greater than two bits use format 2 or the longer alternatives, 3 or 4. These formats support longer payloads such as CQI, multiple HARQs, or a combination of different payloads. Note that UCI payloads can vastly exceed 100 bits when detailed CQI reports are included.

You will often hear that control uses polar coding, but this is only true, strictly speaking, for payloads greater than 11 bits. For shorter payloads, other schemes are used including repetition, simplex code, and Reed Muller code. This slide shows an overview of the PUCCH processing chain for different PUCCH formats. Of course, the chain is quite different for formats that support one or two bits, which is the first two, than for formats that support many bits. Those latter ones include coding and scrambling, whereas coding and randomization are taken care of by means of predefined sequences for the first two chains. The highest level modulation for any PUCCH is QPSK.

Here you can see code from MathWorks 5G Toolbox that implements the PUCCH chain for format 4, the most complicated chain from the previous slide. You can recognize steps such as scrambling, modulation, spreading, and transform precoding. The code also provides all necessary references to 3GPP standard documents. The first content type for UCI is scheduling requests.

Scheduling requests are the means for UE to request access to uplink resources not yet scheduled for transmission. The scheduling requests can be carried in one of two ways, through PUCCH or through RACH. The network may set up semi-statically allocated resources for the UE to send PUCCH, and the UE can use those resources to request data capacity on the uplink. If such resources have not been set up, then the UE must go through the contention-based and slower RACH process to request resources.

As explained in the episode of this "5G Explained" series about downlink control transmission, a downlink control information carrying and uplink grant comes in response to a scheduling request from the UE. When the receives the scheduling requests, it makes all the decisions about when and how the UE should transmit the data that is ready for transmission. Those parameters include--besides the time and frequency, location, and modulation and coding scheme--other information such as precoding, which comes in the form of an index that points to a table of possible precoding matrices. After decoding the control information for the uplink grant, the UE transmits uplink data according to those parameters.

The second type of UCI content is CQI or channel quality indicators. Such information is transmitted in response to a request from the [INAUDIBLE] These requests can be a onetime request, a semi-periodic, or a periodic request, as shown in more detail in another episode of this "5G Explained" series about channel sounding.

The third type of UCI content is ACK/NACK for HARQ. Each transport block transmitted on the downlink has to be acknowledged by the UE. The positive or negative acknowledgment determines whether a block has to be retransmitted at the physical layer or, if the maximum number of transmission has been already reached, skipped for higher layers to decide how to handle the failure. This concludes this episode of the "5G Explained" series on uplink control transmission.